1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an electrically-driven liquid crystal lens, in which a lens, realized based on alignment of liquid crystals, can achieve a gentle parabolic lens surface via a change in electrode configuration, thereby being capable of reducing a cell gap of a liquid crystal layer and achieving a stable profile even when applied to large-area display devices, and a stereoscopic display device using the same.
2. Discussion of the Related Art
At present, services for rapid dissemination of information, constructed on the basis of high-speed information communication networks, have developed from a simple “listening and speaking” service, such as current telephones, to a “watching and listening” multimedia type service on the basis of digital terminals used for high-speed processing of characters, voice and images, and are expected to be ultimately developed into cyberspace 3-dimensional stereoscopic information communication services enabling virtual reality and stereoscopic viewing free from the restrains of time and space.
In general, stereoscopic images representing 3-dimensions are realized based on the principle of stereo-vision via the viewer's eyes. However, since the viewer's eyes are spaced apart from each other by about 65 mm, i.e. have a binocular parallax, the left and right eyes perceive slightly different images due to a positional difference therebetween. Such a difference between images due to the positional difference of the eyes is called binocular disparity. A 3-dimensional stereoscopic image display device is designed on the basis of binocular disparity, allowing the left eye to view only an image for the left eye and the right eye to view only an image for the right eye.
Specifically, the left and right eyes view different two-dimensional images, respectively. If the two different images are transmitted to the brain through the retina, the brain accurately fuses the images, giving the impression of a real 3-dimensional image. This ability is conventionally called stereography, and a display device utilizing stereography is called a stereoscopic display device.
Meanwhile, stereoscopic display devices can be classified according to constituent components of a lens provided to reproduce a 3-dimensional image. For example, a lens, constructed using a liquid crystal layer, is called a liquid crystal lens, which is driven by an electric field. Hereinafter, this kind of lens will be referred to as an electrically-driven liquid crystal lens.
Conventionally, a liquid crystal display device includes two electrodes opposite each other, and a liquid crystal layer formed between the two electrodes. Liquid crystal molecules of the liquid crystal layer are driven by an electric field generated when a voltage is applied to the two electrodes. The liquid crystal molecules have polarization and optical anisotropy properties. Here, according to the polarization property, when a liquid crystal molecule is placed within an electric field, electric charges in the liquid crystal molecule are gathered to opposite sides of the liquid crystal molecule, whereby a molecular arrangement direction is altered according to an applied electric field. The optical anisotropy property is that, owing to an elongated configuration of liquid crystal molecules and the above-described molecular arrangement direction, the path of light to be emitted or polarization conditions of the emitted light is changed according to the incidence direction of incident light or polarization conditions of the incident light.
Accordingly, the liquid crystal layer possesses a difference of transmissivity by voltages applied to the two electrodes, and an image can be displayed using the transmissivity difference of pixels.
Recently, there has been developed an electrically-driven liquid crystal lens in which a liquid crystal layer serves as a lens using the above-described properties of liquid crystal molecules.
Specifically, a lens controls the path of incident light according to a given position using a difference between an index of refraction of a lens constituent material and an index of refraction of air. If different voltages are applied to different positions of the electrodes to drive the liquid crystal layer by different electric fields, the incident light introduced into the liquid crystal layer causes different phase variations at different positions, and as a result, the liquid crystal layer can control the path of incident light in the same manner as an actual lens.
Hereinafter, a conventional electrically-driven liquid crystal lens will be described with reference to the accompanying drawings.
FIG. 1 is a sectional view illustrating a conventional electrically-driven liquid crystal lens, and FIG. 2 is a graph illustrating electric potential distribution upon formation of the electrically-driven liquid crystal lens shown in FIG. 1 after voltages are applied to the liquid crystal lens.
As shown in FIG. 1, the conventional electrically-driven liquid crystal lens includes first and second substrates 10 and 20 arranged opposite each other, and a liquid crystal layer 30 formed between the first substrate 10 and the second substrate 20.
Here, first electrodes 11 are formed on the first substrate 10 such that they are spaced apart from one another by a first interval. In these neighboring first electrodes 11, a distance from the center of one of the first electrodes 11 to the center of the next first electrode 11 is called pitch. Repeating the same pitch for each of the first electrodes results in a pattern.
A second electrode 21 is formed on the entire surface of the second substrate 20 opposite the first substrate 10.
The first and second electrodes 11 and 21 are made of transparent metals. The liquid crystal layer 30 is formed in a space between the first electrodes 11 and the second electrode 21. Liquid crystal molecules constituting the liquid crystal layer 30 have a property of responding to the strength and distribution of an electric field, and thus, have a phase distribution similar to the electrically-driven liquid crystal lens as shown in FIG. 2.
The above-described electrically-driven liquid crystal lens is obtained under the condition of applying a high voltage to the first electrode 11 and grounding the second electrode 21. Under these voltage conditions, a vertical electric field is strongest at the center of the first electrode 11, and the strength of the vertical electric field decreases away from the first electrode 11. Thereby, when the liquid crystal molecules constituting the liquid crystal layer 30 have a positive dielectric constant anisotropy, the liquid crystal molecules are arranged according to the electric field in such a way that they are upright at the center of the first electrode 11 and tilt closer to a horizontal plane with increasing distance from the first electrode 11. As a result, in view of light transmission, an optical path is shortened at the center of the first electrode 11, and is lengthened with increasing distance from the first electrode 11, as shown in FIG. 2. Representing the length variation of the optical path using a phase plane, the electrically-driven liquid crystal lens has light transmission effects similar to a parabolic lens having a parabolic lens surface.
Here, the second electrode 21 causes behavior of an electrically-driven liquid crystal lens, whereby an index of refraction thereof generally takes the form of a spatial parabolic function, and the first electrodes 11 are arranged to define edge regions of the lens.
In this case, a relatively higher voltage is applied to the first electrodes 11 than the second electrode 21. Therefore, as shown in FIG. 2, an electric potential difference occurs between the first electrodes 11 and the second electrode 21. In particular, a steep side electric field is generated around the first electrodes 11. As a result, since liquid crystal molecules cannot achieve a gentle distribution and have a slightly distorted distribution, the conventional electrically-driven liquid crystal lens has characteristics in that an index of refraction of the liquid crystal molecules cannot achieve parabolic spatial distribution or movement of the liquid crystal molecules is excessively sensitive to voltage variation.
The above-described conventional electrically-driven liquid crystal lens can be manufactured by forming electrodes on both substrates, respectively, with liquid crystals interposed therebetween and applying voltages to the electrodes, without using a lens having a physically formed parabolic lens surface.
However, a liquid crystal display device using the above-described electrically-driven liquid crystal lens has the following problems.
Firstly, since electrodes formed on a lower substrate occupy an extremely partial area of a lens region, a steep side electric field, rather than a gentle electric field, is generated between a lens edge region corresponding to the electrodes and a lens center region. The steep side electric field results in a slightly distorted phase of the electrically-driven liquid crystal lens. In particular, in the electrically-driven liquid crystal lens formed by a liquid crystal electric field, as the pitch of the lens region is increased, the number of electrodes, to which a high voltage is applied, is limited. Therefore, the lens region has an insufficient electric field between the electrodes, to which a high voltage is applied, and an upper substrate opposite the electrodes. As a result, formation of the electrically-driven liquid crystal lens having a gentle parabolic lens surface suitable to achieve the same optical effects as an actual lens is extremely difficult.
Secondly, in the case of a large-area display device, the lens center region, which is distant from the lens edge region where the electrodes, to which a high-voltage is applied, are located, is essentially unaffected by an electric field, complicating alignment control of liquid crystals using the electric field and resulting in a serious distortion in lens shape based on the electric field. As occasion demands, when control in the lens center region is difficult or impossible, the resulting electrically-driven liquid crystal lens has a discontinuous lens profile, which makes the electrically-driven liquid crystal lens ineffective as a lens.
Thirdly, the electrically-driven liquid crystal lens, constructed by a vertical electric field between the electrodes to which a single high voltage is applied and an electrode formed on the entire surface of the substrate opposite to the electrodes to which the high voltage is applied, sags. Therefore, the electrically-driven liquid crystal lens must have upper and lower margins, and requires an enormous amount of liquid crystals. There exists an urgent need to remedy this problem.